The present invention concerns a method and apparatus for monitoring an NOx storage catalytic converter located downstream of a lean-running internal combustion engine. NOx storage catalytic converters consist of a conventional 3-way coating that is augmented by an NOx storage component. Such converters store the nitrogen oxides contained in the exhaust by forming nitrates during lean operation, and convert them into harmless nitrogen during intermittent catalyst regeneration under reducing conditions, during which process the converters are deliberately emptied and regain essentially their full absorption capacity for nitrogen oxides. This capacity decreases continuously with increasing nitrogen oxide loading during the lean phase.
In contrast, the NOx absorption capacity or storage efficiency of a thermally damaged or poisoned NOx storage catalytic converter is lower, which is reflected during lean operation in a more rapidly rising NOx concentration or NOx mass downstream of the storage catalytic converter, with the result that proper exhaust gas purification is no longer achieved.
Consequently, monitoring processes for verifying the proper functioning of the catalytic converters are of great interest for practical use of NOx storage catalytic converters. Existing conventional catalytic converter diagnostic procedures, in which the oxygen storage capacity of the catalyst is determined by means of lambda sensors, and is correlated with the catalytic converter throughput and emission characteristics, are not suitable for lean operation. In an operating mode with xcex greater than 1, which represents the operating mode that determines emissions for future lean mix concepts, oxygen sensors do not produce a usable signal with adequate resolution due to the constant presence of residual oxygen in the exhaust.
The object of the present invention is thus to produce a monitoring or diagnostic method for an NOx storage catalytic converter located downstream of a lean-running internal combustion engine which permits the reliable and rapid detection of a faulty or poisoned catalyst and a resulting reduction in catalyst activity or storage efficiency, so that proper functioning of the catalytic converter is always ensured. The object also is to produce an exhaust gas purification device for carrying out this method which includes an NOx storage catalytic converter.
In the process according to the invention, the NOx concentrations are measured in the exhaust gas before and after the NOx storage catalytic converter and are converted into NOx mass flows. The NOx storage efficiency, which is the actual quantity of interest, is then calculated from these values and is compared to a threshold value that indicates a storage catalyst that is faulty or no longer adequately functional; if the threshold is not met, a catalyst failure is reported.
However, since a reduction in catalyst activity or storage efficiency can also result from excessively high nitrogen oxide loading and/or reversible sulfur poisoning, for example, it is preferable for at least one NOx regeneration and/or desulfating to be performed when the threshold is not met and for the storage efficiency to be measured again. Only if the predetermined threshold is not met again after regeneration is irreversible catalyst damage assumed to have occurred, which is then reported as such.
In a preferred embodiment, before and during the catalyst monitoring, conformity with predefined operating conditions is verified, which may include for example verifying the catalyst loading, catalyst temperature and the lambda value, which among other things ensures that the catalyst monitoring is performed only during lean operation and in a temperature range in which the catalyst is active with regard to NOx storage. Hence the catalyst temperature must preferably lie in a predetermined temperature range, more particularly between 200 and 600xc2x0 C., which range depends upon the catalyst system used and more particularly upon the catalyst coating. In addition, the NOx loading of the catalyst must be below a predetermined threshold value, which preferably corresponds to a fully regenerated catalyst. Moreover, the lambda value upstream and/or downstream of the storage catalytic converter must be  greater than 1, in which case the selection of lean operation preferably must be indicated by a status bit in the engine control system.
In the preferred embodiment if these conditions are not met, catalyst monitoring is not performed, or monitoring that has already begun is terminated or at least interrupted.
At the beginning of the monitoring process, it is preferable to start a timing function on a timer that is compared to a predetermined monitoring or diagnostic time. Once this time is reached, the measured storage efficiency is stored; the number of stored values preferably are counted, and averaged once a predetermined number of stored values has been reached. The averaged value is then finally compared to the aforementioned threshold value.
In a further preferred embodiment, to compensate for the dependency of the storage efficiency on catalyst loading and space velocity, the exhaust gas concentration measured downstream of the NOx storage catalytic converter and/or the NOx mass flow determined therefrom, are evaluated or weighted as a function of these two quantities.
The storage efficiency is preferably also weighted with the averaged catalyst temperature determined during the monitoring time, which is determined by temperature sensors arranged upstream and downstream of the storage catalytic converter or is calculated through modeling in an associated control unit.
Another preferred possibility is to measure the NOx storage efficiency for several different temperatures as a function of the average catalyst temperature and to store it in a characteristic curve or performance graph, from which aged, poisoned or faulty catalysts can be easily detected and reliably diagnosed on the basis of their characteristic changes. The characteristic changes in the performance graph can include a reduction in storage efficiency at specific temperatures, a corresponding change in the average value of the reduction in storage efficiency over a specific temperature range, or a change in the catalyst temperature at which a specific efficiency is achieved.
The lambda value and the NOx concentration in the exhaust flowing into or out of the NOx storage catalytic converter are preferably determined by means of multifunction sensors, arranged upstream and/or downstream, which include NOx-sensitive and oxygen-sensitive measurement devices.
An exhaust gas device in accordance with the invention for carrying out this process includes, an NOx storage catalytic converter with lambda sensors arranged upstream and downstream thereof to determine the oxygen content of the exhaust flowing into and out of the catalytic converter. In addition, NOx sensors are arranged upstream and downstream of the NOx storage catalytic converter to determine the NOx concentrations in the exhaust gas. Alternatively, the NOx concentration in the exhaust gas flowing in can also be determined by an associated modeling system, so that the upstream NOx sensor can be omitted.
Preferred embodiments of this exhaust gas purification device include, for example, a temperature sensor arranged upstream and/or downstream or at least one multifunction sensor with an integrated NOx-sensitive and oxygen-sensitive measurement device.
It is advantageous for the sensors that are used to be adapted with respect to zero point drift and characteristic curve offset.
The exhaust gas purification device in accordance with the invention also includes an analysis and/or control unit to analyze the sensor signals and/or control the diagnostic process described above.
Further characteristics and advantages of the process in accordance with the invention and of the exhaust gas purification device in accordance with the invention for performing the process can be found not only in the associated claimsxe2x80x94individually and/or in combinationxe2x80x94but also in the description below of preferred example embodiments in conjunction with the associated drawings.